1. Field of the Invention
The present invention relates to a vehicle exhaust apparatus and a motorcycle equipped therewith, particularly to the vehicle exhaust apparatus constituting an exhaust passage to an exhaust muffler from plural exhaust pipes connected to a multi-cylinder engine, and the motorcycle equipped with the vehicle exhaust apparatus.
2. Description of the Related Art
Some vehicle exhaust apparatuses connected to a multi-cylinder engine have a structure in which individual cylinder exhaust pipes connected to cylinders are independently connected to respective exhaust mufflers. From the standpoint of compact piping space and exhaust gas pulsation attenuation, frequently the vehicle exhaust apparatus has a structure in which plural exhaust passages are collected at the midpoint of an exhaust passage or a structure in which the exhaust passage is rebranched into plural exhaust passages after they are collected.
FIGS. 18 and 19 show an example of a conventional motorcycle exhaust apparatus connected to a 4-cylinder 4-cycle engine 110. Referring to FIG. 18, which is a plan view, first, second, third, and fourth individual cylinder exhaust pipes 120-1, 120-2, 120-3, and 120-4 are connected to four cylinders of the engine 110 respectively, right and left mufflers 121 are arranged on both sides of a rear wheel 117, and the four individual cylinder exhaust pipes 120-1, 120-2, 120-3, and 120-4 and the two exhaust mufflers 121 are connected through upper and lower first exhaust collector pipes 123, a second exhaust collector pipe 124, right and left branched exhaust pipes 126, and right and left rear exhaust pipes 127.
Among the four individual cylinder exhaust pipes 120-1, 120-2, 120-3, and 120-4, the first individual cylinder exhaust pipe 120-1 and the fourth individual cylinder exhaust pipe 120-4 are collected to one upper exhaust passage by the upper first exhaust collector pipe 123. As shown in FIG. 19, which is a side view, the second individual cylinder exhaust pipe 120-2 and the third individual cylinder exhaust pipe 120-3 are collected to one lower exhaust passage by the lower first exhaust collector pipe 123. In the second exhaust collector pipe 124, the exhaust passage of the upper first exhaust collector pipe 123 and the exhaust passage of the lower first exhaust collector pipe 123 are collected in one exhaust passage. In the branched exhaust pipe 126, the exhaust passage of the second exhaust collector pipe 124 is rebranched into the right and left rear exhaust pipes 127. As for examples of conventional art, Japanese Utility Model Laid-Open No. S63-130618 discloses an exhaust apparatus similar to the above-described exhaust apparatus, and Japanese Utility Model Laid-Open No. H6-73319 discloses an exhaust apparatus in which the four individual cylinder exhaust pipes are finally collected in one exhaust muffler.
In the exhaust apparatus shown in FIGS. 18 and 19, an exhaust gasflow sectional area of a rear end portion 125 of the second exhaust collector pipe 124 is formed so as to be substantially equal to or smaller than the exhaust gasflow sectional area of a front half portion of the second exhaust collector pipe 124. In such exhaust apparatuses, a valley of an engine torque may be generated during acceleration in a low-speed range of the engine.
FIG. 17 shows a relationship between the engine speed and the engine torque. A graph X1 indicated by a broken line shows an engine torque change (torque curve) in the engine equipped with the exhaust apparatus of FIGS. 18 and 19. Referring to FIG. 17, in the conventional exhaust apparatus, during the acceleration in the low-speed range near the engine speed N1, there is generated a phenomenon called “torque valley” in which the torque is temporarily rapidly decreased and then raised as shown by a point P1. This phenomenon gives a rider a feeling that power of the engine is temporarily lost. That is, during the acceleration operation, because the torque is temporarily decreased against the rider's operation of accelerating, the operation does not feel comfortable.
In the conventional exhaust apparatuses disclosed in Japanese Utility Model Laid-Open Nos. S63-130618 and H6-73319, the collector portions of the individual cylinder exhaust pipes are each formed in an expanded shape such as a cylindrical shape to suppress the generation of the torque valley in the low-speed range. However, the expanded collector portion has little effect of eliminating the torque valley in the low-speed range.